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They observed that after amber tools were cleaned with animal fur, they

would attract small bits of dust and

feathers.

The earliest account of electric phenomena

comes from the Ancient Greeks.

The word electricity comes from the Greek

word for amber – electron.

In the 1700’s, an American scientist conducted extensive research in these

phenomena.

He sold many of his possessions to fund his work.

Benjamin Franklin recorded that when an amber rod was rubbed with fur, the amber would attract the fur that it was rubbed with.

However, he also discovered that two amber rods rubbed with fur would repel one another.

He was able to achieve similar results with a glass rod rubbed with silk, but with one major difference:

The rubbed glass rod and the rubbed amber rod repel one

another too!

He concluded the following:

Ben Franklin’s Conclusions

1.This was not the same phenomenon as magnetism!

These objects did not have two poles!

Rather, rubbing the materials gave them a property which Franklin called electric charge.

Ben Franklin’s Conclusions

Several years later, while Benjamin Franklin was helping to build our nation, a French

scientist named Charles-Augustin de Coulomb ventured a step further.

“The attractive force between two oppositely charged

spheres is proportional to the product of the quantities of

charge on the spheres and is inversely proportional to the

square of the distance between the spheres”

(Originally in French)

Coulomb’s Lawq1 q2

d

q1 is the magnitude of the charge of object 1.q2 is the magnitude of the charge of object 2.

k is a constant known as Coulomb’s constant (k = 9 x 109 Nm2/C2)

If the two objects have the same type of charge, they will repel one another.

If the two objects have different types of charge, they will attract one another.

can get you the magnitude of the electric forces exerted on each object, and you can use some reasoning to find the direction.

+ +

_ _

_+

The unit of electric charge is the Coulomb (C), in honor of Charles

Coulomb.

Electrostatic Conceptual Question

Two isolated charges, +q and ‑2q, are 2 centimeters apart. If F is the magnitude of the force acting on charge ‑2q, what are the magnitude and direction of the force acting on charge +q?

Magnitude Direction

(A) F/2 Toward charge ‑2q (B) 2F Away from charge ‑2q (C) F Toward charge ‑2q (D) F Away from charge ‑2q (E) 2F Toward charge ‑2q

Newton’s Third Law!

SolutionTwo isolated charges, +q and ‑2q, are 2 centimeters apart. If F is the magnitude of the force acting on charge ‑2q, what are the magnitude and direction of the force acting on charge +q?

Magnitude Direction

(A) F/2 Toward charge ‑2q (B) 2F Away from charge ‑2q (C) F Toward charge ‑2q (D) F Away from charge ‑2q (E) 2F Toward charge ‑2q

Whiteboard Reasoning with Coulomb’s Law

Two charged spheres are held near one another, as shown above.

If one of the spheres is replaced with a sphere of charge +2q, and they are brought to half the distance apart, what will be the resulting force exerted on each of the spheres?

+q +qF F

d

+q +2q

d/2

Whiteboard: The Hydrogen AtomA hydrogen atom is composed of a proton and an electron.

qelectron = -1.6 x 10-19 C

qproton = 1.6 x 10-19 C

k = 9 x 109 Nm2/C2

mproton = 1.67 x 10-27 kg

melectron = 9.1 x 10-31 kg

G = 6.67 x 10-11 Nm2/kg2

Calculate the ratio of the strength of the electric force that each particle exerts on the other to the strength of the gravitational force that particle exerts on the other.

= 2.27 x 1039 times as strong!!!

The electrical force between the particles is much, much stronger than the gravitational force!

The Principle of Superposition

The net force exerted on an object is the vector sum of the individual forces exerted on it. Treat each force separately using

Coulomb’s Law, and then add them as vectors!

+

+ +

Final Whiteboard: Superimpose!

Determine the magnitude and direction of the net force exerted on q1 in the configuration shown below.

+

_ +

3 cm

3 cm

3 cm

q1 = 6 x 10-7 C

q2 = -6 x 10-7 C q1 = 6 x 10-7 C

Net Result

+

_ +

Fnet = 3.6 N

60°60°

60°60°

60°

The y-components of the forces negate one another, and the x-components add to one another!